Apparatus for treating fuel vapor to prevent fuel vapor inside a fuel tank from diffusing into the atmosphere (referring to Japanese Unexamined Patent Publication No. 62-7962), have involved temporarily absorbing the fuel vapor produced inside the fuel tank into a canister, and then purging the absorbed fuel vapor and supplying the purge air through a purge passage to the engine intake system.
With such an apparatus the purge air from a canister is supplied to the engine intake system, with the normal supply mixture. As a fixed quantity of purge air is supplied irrespective of the engine operating conditions, there is the likelihood of a large discrepancy in the air-fuel ratio.
To compensate, the quantity of purge air is changed in accordance with engine operating conditions such as engine load. To achieve this, the purge air quantity is adjusted by duty controlling the ON/OFF switching of a purge control solenoid valve that drives a valve for opening and closing the purge passage, according to a duty ratio corresponding to the purge flow quantity required for the operating conditions.
However, with the duty control of the purge control solenoid valve, when the frequency of the duty control is low, purge air pulsations occur with the opening/closing of the purge passage. As a result, variations occur in the purge air quantity drawn into the respective cylinders causing variations in the air-fuel ratio between the cylinders.
So that the purge air is drawn in uniformly into the respective cylinders, the frequency of the duty control should be as high as possible. However, if a high frequency is used, then the power supply time becomes shorter so that the flow quantity region, where there is a drop in control accuracy due to the valve opening delay time, is enlarged compared to when a low frequency is used. As a result, high accuracy flow quantity adjustment cannot be made in the low flow quantity region.
More specifically, while the solenoid valve has a constant valve opening delay time, when duty control is carried out at a high frequency, the power supply time (valve opening time) is shortened compared to that for control at a low frequency, so that the proportion of the power supply time control region occupied by the valve opening delay time is increased. Therefore, when adjusting a low flow quantity at a high frequency, the power supply time within the valve opening delay time during which the air quantity is unstable is given, making it difficult to adjust a low flow quantity to a high accuracy.
Moreover, with a construction wherein a high frequency that can reliably avoid purge air pulsations is used indiscriminately, since the ON/OFF switching per unit time is increased significantly, then there is the likelihood of deterioration in the life of the valve seat components.